This invention relates generally to a means for reducing flutter in tape recording systems and particularly to an all digital device for reducing the time base error in magnetic tape recorders having data recorded by FM techniques.
The flutter that results from tape speed variations and the record/reproduce process in magnetic tape recording has historically been a problem. In using tape recorders for the storage and playback of pulse type data, variations in the recorder and playback speed result in spurious signals commonly known as flutter. The speed variations may be caused by tape slippage, mechanical vibrations, imperfect mechanical motion, changes in the dimension of the recording medium or variations in the speed of the recorder drive systems. Flutter is a relatively slow variation in the speed of the tape past the recording or the playback head. Because speed is one of the factors that determine the timing of the data signal to be reproduced, any variation in this speed will produce a variation in the reproduced signal.
In a typical FM tape recording system using a reference tone track for speed control, a highly stable tone is recorded on one track at the same time the data signal is recorded on a second track. The highly stable tone is provided by a reference oscillator. The reference tone is commonly used as a control signal in an electromechanical servo system to maintain a rotating type drive capstan at a constant speed during the recording process. Typically, a tachometer coupled to the recorder capstan provides the necessary feedback signal during recording. During the reproduction process a reference tone, which has the same characteristics as that used in recording, together with the output of the reference track is applied to a phase lock servo loop which attempts to control the tape speed so that the tone obtained from the reference track is a time replica of the original tone applied to the reference track during recording. Accordingly, if the servo technique were errorless the playback data signal would also be a time replica of the original signal. The instantaneous time base error (TBE) can readily be observed by taking a measurement during playback of the time difference between the zero crossings of the signal from the reference oscillator or tone generator and the signal obtained from the reference track. It has been observed in some reproducers running at a tape speed of 7.5 inches per second that time base errors of up to 2.5 microseconds (.mu.sec.) and greater remain after correction by the electromechanical servo, depending upon the particular tape recorder being utilized.
Prior efforts to minimize such errors have included methods for keeping speed variations to a minimum by sophisticated electromechanical servo techniques with well designed and accurately machined tape transport mechanisms. A device for flutter reduction which utilizes hybrid circuitry composed of both analog and digital electronic circuits is disclosed in patent application Ser. NO. 512,835 by DeFrancesco et al. In analog circuits that are used for time base correction, the zero crossings of the data signal are advanced or delayed by having a voltage proportional to the TBE control the pulse width of a one-shot multivibrator or the "on time" of a triggered linear sweep circuit. In particular, the error correction portion of the hybrid device is entirely analog and which, accordingly, operates on varying quantities of ever changing voltages and currents. Such analog circuits lack stability due to the inherent drift of components and operating parameters.